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adamc@62
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1 (* Copyright (c) 2008, Adam Chlipala
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2 *
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3 * This work is licensed under a
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4 * Creative Commons Attribution-Noncommercial-No Derivative Works 3.0
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5 * Unported License.
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6 * The license text is available at:
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7 * http://creativecommons.org/licenses/by-nc-nd/3.0/
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8 *)
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9
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10 (* begin hide *)
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11 Require Import List.
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12
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13 Require Import Tactics.
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14
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15 Set Implicit Arguments.
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16 (* end hide *)
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17
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18
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19 (** %\chapter{Infinite Data and Proofs}% *)
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20
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21 (** In lazy functional programming languages like Haskell, infinite data structures are everywhere. Infinite lists and more exotic datatypes provide convenient abstractions for communication between parts of a program. Achieving similar convenience without infinite lazy structures would, in many cases, require acrobatic inversions of control flow.
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22
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23 Laziness is easy to implement in Haskell, where all the definitions in a program may be thought of as mutually recursive. In such an unconstrained setting, it is easy to implement an infinite loop when you really meant to build an infinite list, where any finite prefix of the list should be forceable in finite time. Haskell programmers learn how to avoid such slip-ups. In Coq, such a laissez-faire policy is not good enough.
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24
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25 We spent some time in the last chapter discussing the Curry-Howard isomorphism, where proofs are identified with functional programs. In such a setting, infinite loops, intended or otherwise, are disastrous. If Coq allowed the full breadth of definitions that Haskell did, we could code up an infinite loop and use it to prove any proposition vacuously. That is, the addition of general recursion would make CIC %\textit{%#<i>#inconsistent#</i>#%}%.
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26
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27 There are also algorithmic considerations that make universal termination very desirable. We have seen how tactics like [reflexivity] compare terms up to equivalence under computational rules. Calls to recursive, pattern-matching functions are simplified automatically, with no need for explicit proof steps. It would be very hard to hold onto that kind of benefit if it became possible to write non-terminating programs; we would be running smack into the halting problem.
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28
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29 One solution is to use types to contain the possibility of non-termination. For instance, we can create a "non-termination monad," inside which we must write all of our general-recursive programs. In later chapters, we will spend some time on this idea and its applications. For now, we will just say that it is a heavyweight solution, and so we would like to avoid it whenever possible.
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30
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31 Luckily, Coq has special support for a class of lazy data structures that happens to contain most examples found in Haskell. That mechanism, %\textit{%#<i>#co-inductive types#</i>#%}%, is the subject of this chapter. *)
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32
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33
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34 (** * Computing with Infinite Data *)
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35
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36 (** Let us begin with the most basic type of infinite data, %\textit{%#<i>#streams#</i>#%}%, or lazy lists. *)
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37
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38 Section stream.
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39 Variable A : Set.
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40
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41 CoInductive stream : Set :=
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42 | Cons : A -> stream -> stream.
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43 End stream.
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44
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45 (** The definition is surprisingly simple. Starting from the definition of [list], we just need to change the keyword [Inductive] to [CoInductive]. We could have left a [Nil] constructor in our definition, but we will leave it out to force all of our streams to be infinite.
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46
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47 How do we write down a stream constant? Obviously simple application of constructors is not good enough, since we could only denote finite objects that way. Rather, whereas recursive definitions were necessary to %\textit{%#<i>#use#</i>#%}% values of recursive inductive types effectively, here we find that we need %\textit{%#<i>#co-recursive definitions#</i>#%}% to %\textit{%#<i>#build#</i>#%}% values of co-inductive types effectively.
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48
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49 We can define a stream consisting only of zeroes. *)
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50
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51 CoFixpoint zeroes : stream nat := Cons 0 zeroes.
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52
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53 (** We can also define a stream that alternates between [true] and [false]. *)
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54
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55 CoFixpoint trues : stream bool := Cons true falses
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56 with falses : stream bool := Cons false trues.
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57
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58 (** Co-inductive values are fair game as arguments to recursive functions, and we can use that fact to write a function to take a finite approximation of a stream. *)
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59
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60 Fixpoint approx A (s : stream A) (n : nat) {struct n} : list A :=
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61 match n with
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62 | O => nil
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63 | S n' =>
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64 match s with
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65 | Cons h t => h :: approx t n'
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66 end
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67 end.
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68
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69 Eval simpl in approx zeroes 10.
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70 (** [[
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71
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72 = 0 :: 0 :: 0 :: 0 :: 0 :: 0 :: 0 :: 0 :: 0 :: 0 :: nil
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73 : list nat
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74 ]] *)
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75 Eval simpl in approx trues 10.
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76 (** [[
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77
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78 = true
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79 :: false
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80 :: true
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81 :: false
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82 :: true :: false :: true :: false :: true :: false :: nil
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83 : list bool
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84 ]] *)
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85
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86 (* begin thide *)
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87 (** So far, it looks like co-inductive types might be a magic bullet, allowing us to import all of the Haskeller's usual tricks. However, there are important restrictions that are dual to the restrictions on the use of inductive types. Fixpoints %\textit{%#<i>#consume#</i>#%}% values of inductive types, with restrictions on which %\textit{%#<i>#arguments#</i>#%}% may be passed in recursive calls. Dually, co-fixpoints %\textit{%#<i>#produce#</i>#%}% values of co-inductive types, with restrictions on what may be done with the %\textit{%#<i>#results#</i>#%}% of co-recursive calls.
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88
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89 The restriction for co-inductive types shows up as the %\textit{%#<i>#guardedness condition#</i>#%}%, and it can be broken into two parts. First, consider this stream definition, which would be legal in Haskell.
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90
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91 [[
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92 (* end thide *)
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93 CoFixpoint looper : stream nat := looper.
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94 (* begin thide *)
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95 [[
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96 Error:
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97 Recursive definition of looper is ill-formed.
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98 In environment
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99 looper : stream nat
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100
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101 unguarded recursive call in "looper"
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102 *)
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103 (* end thide *)
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104
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105 (** The rule we have run afoul of here is that %\textit{%#<i>#every co-recursive call must be guarded by a constructor#</i>#%}%; that is, every co-recursive call must be a direct argument to a constructor of the co-inductive type we are generating. It is a good thing that this rule is enforced. If the definition of [looper] were accepted, our [approx] function would run forever when passed [looper], and we would have fallen into inconsistency.
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106
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107 The second rule of guardedness is easiest to see by first introducing a more complicated, but legal, co-fixpoint. *)
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108
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109 Section map.
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110 Variables A B : Set.
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111 Variable f : A -> B.
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112
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113 CoFixpoint map (s : stream A) : stream B :=
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114 match s with
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115 | Cons h t => Cons (f h) (map t)
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116 end.
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117 End map.
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118
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119 (** This code is a literal copy of that for the list [map] function, with the [Nil] case removed and [Fixpoint] changed to [CoFixpoint]. Many other standard functions on lazy data structures can be implemented just as easily. Some, like [filter], cannot be implemented. Since the predicate passed to [filter] may reject every element of the stream, we cannot satisfy even the first guardedness condition.
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120
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121 The second condition is subtler. To illustrate it, we start off with another co-recursive function definition that %\textit{%#<i>#is#</i>#%}% legal. The function [interleaves] takes two streams and produces a new stream that alternates between their elements. *)
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122
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123 Section interleave.
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124 Variable A : Set.
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125
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126 CoFixpoint interleave (s1 s2 : stream A) : stream A :=
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127 match s1, s2 with
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128 | Cons h1 t1, Cons h2 t2 => Cons h1 (Cons h2 (interleave t1 t2))
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129 end.
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130 End interleave.
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131
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132 (** Now say we want to write a weird stuttering version of [map] that repeats elements in a particular way, based on interleaving. *)
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133
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134 Section map'.
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135 Variables A B : Set.
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136 Variable f : A -> B.
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137
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138 (* begin thide *)
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139 (** [[
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140 (* end thide *)
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141
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142 CoFixpoint map' (s : stream A) : stream B :=
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143 match s with
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144 | Cons h t => interleave (Cons (f h) (map' s)) (Cons (f h) (map' s))
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145 end.
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146 (* begin thide *)
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147 *)
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148
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149 (** We get another error message about an unguarded recursive call. This is because we are violating the second guardedness condition, which says that, not only must co-recursive calls be arguments to constructors, there must also %\textit{%#<i>#not be anything but [match]es and calls to constructors of the same co-inductive type#</i>#%}% wrapped around these immediate uses of co-recursive calls. The actual implemented rule for guardedness is a little more lenient than what we have just stated, but you can count on the illegality of any exception that would enhance the expressive power of co-recursion.
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150
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151 Why enforce a rule like this? Imagine that, instead of [interleave], we had called some other, less well-behaved function on streams. Perhaps this other function might be defined mutually with [map']. It might deconstruct its first argument, retrieving [map' s] from within [Cons (f h) (map' s)]. Next it might try a [match] on this retrieved value, which amounts to deconstructing [map' s]. To figure out how this [match] turns out, we need to know the top-level structure of [map' s], but this is exactly what we started out trying to determine! We run into a loop in the evaluation process, and we have reached a witness of inconsistency if we are evaluating [approx (map' s) 1] for any [s]. *)
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152 (* end thide *)
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153 End map'.
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154
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155
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156 (** * Infinite Proofs *)
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157
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158 (** Let us say we want to give two different definitions of a stream of all ones, and then we want to prove that they are equivalent. *)
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159
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160 CoFixpoint ones : stream nat := Cons 1 ones.
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161 Definition ones' := map S zeroes.
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162
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163 (** The obvious statement of the equality is this: *)
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164
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165 Theorem ones_eq : ones = ones'.
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166
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167 (** However, faced with the initial subgoal, it is not at all clear how this theorem can be proved. In fact, it is unprovable. The [eq] predicate that we use is fundamentally limited to equalities that can be demonstrated by finite, syntactic arguments. To prove this equivalence, we will need to introduce a new relation. *)
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168 (* begin thide *)
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169 Abort.
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170
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171 (** Co-inductive datatypes make sense by analogy from Haskell. What we need now is a %\textit{%#<i>#co-inductive proposition#</i>#%}%. That is, we want to define a proposition whose proofs may be infinite, subject to the guardedness condition. The idea of infinite proofs does not show up in usual mathematics, but it can be very useful (unsurprisingly) for reasoning about infinite data structures. Besides examples from Haskell, infinite data and proofs will also turn out to be useful for modelling inherently infinite mathematical objects, like program executions.
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172
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173 We are ready for our first co-inductive predicate. *)
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174
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175 Section stream_eq.
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176 Variable A : Set.
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177
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178 CoInductive stream_eq : stream A -> stream A -> Prop :=
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179 | Stream_eq : forall h t1 t2,
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180 stream_eq t1 t2
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181 -> stream_eq (Cons h t1) (Cons h t2).
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182 End stream_eq.
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183
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184 (** We say that two streams are equal if and only if they have the same heads and their tails are equal. We use the normal finite-syntactic equality for the heads, and we refer to our new equality recursively for the tails.
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185
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186 We can try restating the theorem with [stream_eq]. *)
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187
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188 Theorem ones_eq : stream_eq ones ones'.
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189 (** Coq does not support tactical co-inductive proofs as well as it supports tactical inductive proofs. The usual starting point is the [cofix] tactic, which asks to structure this proof as a co-fixpoint. *)
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190 cofix.
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191 (** [[
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192
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193 ones_eq : stream_eq ones ones'
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194 ============================
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195 stream_eq ones ones'
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196 ]] *)
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197
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198 (** It looks like this proof might be easier than we expected! *)
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199
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200 assumption.
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201 (** [[
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202
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203 Proof completed. *)
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204
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205 (** Unfortunately, we are due for some disappointment in our victory lap. *)
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206
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207 (** [[
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208 Qed.
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209
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210 Error:
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211 Recursive definition of ones_eq is ill-formed.
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212
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213 In environment
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214 ones_eq : stream_eq ones ones'
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215
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216 unguarded recursive call in "ones_eq" *)
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217
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218 (** Via the Curry-Howard correspondence, the same guardedness condition applies to our co-inductive proofs as to our co-inductive data structures. We should be grateful that this proof is rejected, because, if it were not, the same proof structure could be used to prove any co-inductive theorem vacuously, by direct appeal to itself!
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219
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220 Thinking about how Coq would generate a proof term from the proof script above, we see that the problem is that we are violating the first part of the guardedness condition. During our proofs, Coq can help us check whether we have yet gone wrong in this way. We can run the command [Guarded] in any context to see if it is possible to finish the proof in a way that will yield a properly guarded proof term.
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221
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222 [[
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223 Guarded.
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224
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225 Running [Guarded] here gives us the same error message that we got when we tried to run [Qed]. In larger proofs, [Guarded] can be helpful in detecting problems %\textit{%#<i>#before#</i>#%}% we think we are ready to run [Qed].
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226
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227 We need to start the co-induction by applying one of [stream_eq]'s constructors. To do that, we need to know that both arguments to the predicate are [Cons]es. Informally, this is trivial, but [simpl] is not able to help us. *)
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228
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229 Undo.
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230 simpl.
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231 (** [[
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232
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233 ones_eq : stream_eq ones ones'
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234 ============================
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235 stream_eq ones ones'
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236 ]] *)
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237
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238 (** It turns out that we are best served by proving an auxiliary lemma. *)
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239 Abort.
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240
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241 (** First, we need to define a function that seems pointless on first glance. *)
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242
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243 Definition frob A (s : stream A) : stream A :=
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244 match s with
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245 | Cons h t => Cons h t
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246 end.
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247
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248 (** Next, we need to prove a theorem that seems equally pointless. *)
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249
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250 Theorem frob_eq : forall A (s : stream A), s = frob s.
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251 destruct s; reflexivity.
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252 Qed.
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253
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254 (** But, miraculously, this theorem turns out to be just what we needed. *)
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255
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256 Theorem ones_eq : stream_eq ones ones'.
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257 cofix.
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258
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259 (** We can use the theorem to rewrite the two streams. *)
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260 rewrite (frob_eq ones).
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261 rewrite (frob_eq ones').
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262 (** [[
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263
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264 ones_eq : stream_eq ones ones'
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265 ============================
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266 stream_eq (frob ones) (frob ones')
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267 ]] *)
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268
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269 (** Now [simpl] is able to reduce the streams. *)
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adamc@63
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270
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adamc@63
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271 simpl.
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adamc@63
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272 (** [[
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adamc@63
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273
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adamc@63
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274 ones_eq : stream_eq ones ones'
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adamc@63
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275 ============================
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adamc@63
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276 stream_eq (Cons 1 ones)
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adamc@63
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277 (Cons 1
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adamc@63
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278 ((cofix map (s : stream nat) : stream nat :=
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adamc@63
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279 match s with
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adamc@63
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280 | Cons h t => Cons (S h) (map t)
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adamc@63
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281 end) zeroes))
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adamc@63
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282 ]] *)
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adamc@63
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283
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adamc@63
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284 (** Since we have exposed the [Cons] structure of each stream, we can apply the constructor of [stream_eq]. *)
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adamc@63
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285
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adamc@63
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286 constructor.
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adamc@63
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287 (** [[
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adamc@63
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288
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adamc@63
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289 ones_eq : stream_eq ones ones'
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adamc@63
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290 ============================
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adamc@63
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291 stream_eq ones
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adamc@63
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292 ((cofix map (s : stream nat) : stream nat :=
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adamc@63
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293 match s with
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adamc@63
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294 | Cons h t => Cons (S h) (map t)
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adamc@63
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295 end) zeroes)
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adamc@63
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296 ]] *)
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adamc@63
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297
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adamc@63
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298 (** Now, modulo unfolding of the definition of [map], we have matched our assumption. *)
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adamc@63
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299 assumption.
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adamc@63
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300 Qed.
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adamc@63
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301
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adamc@63
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302 (** Why did this silly-looking trick help? The answer has to do with the constraints placed on Coq's evaluation rules by the need for termination. The [cofix]-related restriction that foiled our first attempt at using [simpl] is dual to a restriction for [fix]. In particular, an application of an anonymous [fix] only reduces when the top-level structure of the recursive argument is known. Otherwise, we would be unfolding the recursive definition ad infinitum.
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adamc@63
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303
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adamc@63
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304 Fixpoints only reduce when enough is known about the %\textit{%#<i>#definitions#</i>#%}% of their arguments. Dually, co-fixpoints only reduce when enough is known about %\textit{%#<i>#how their results will be used#</i>#%}%. In particular, a [cofix] is only expanded when it is the discriminee of a [match]. Rewriting with our superficially silly lemma wrapped new [match]es around the two [cofix]es, triggering reduction.
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adamc@63
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305
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adamc@63
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306 If [cofix]es reduced haphazardly, it would be easy to run into infinite loops in evaluation, since we are, after all, building infinite objects.
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adamc@63
|
307
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adamc@63
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308 One common source of difficulty with co-inductive proofs is bad interaction with standard Coq automation machinery. If we try to prove [ones_eq'] with automation, like we have in previous inductive proofs, we get an invalid proof. *)
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adamc@63
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309
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adamc@63
|
310 Theorem ones_eq' : stream_eq ones ones'.
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adamc@63
|
311 cofix; crush.
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adamc@63
|
312 (** [[
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adamc@63
|
313
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adamc@63
|
314 Guarded. *)
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adamc@63
|
315 Abort.
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adamc@68
|
316 (* end thide *)
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adamc@63
|
317
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adamc@63
|
318 (** The standard [auto] machinery sees that our goal matches an assumption and so applies that assumption, even though this violates guardedness. One usually starts a proof like this by [destruct]ing some parameter and running a custom tactic to figure out the first proof rule to apply for each case. Alternatively, there are tricks that can be played with "hiding" the co-inductive hypothesis. We will see examples of effective co-inductive proving in later chapters. *)
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adamc@64
|
319
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adamc@64
|
320
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adamc@64
|
321 (** * Simple Modeling of Non-Terminating Programs *)
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adamc@64
|
322
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adamc@67
|
323 (** We close the chapter with a quick motivating example for more complex uses of co-inductive types. We will define a co-inductive semantics for a simple assembly language and use that semantics to prove that an optimization function is sound. We start by defining types of registers, program labels, and instructions. *)
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adamc@64
|
324
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adamc@64
|
325 Inductive reg : Set := R1 | R2.
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adamc@64
|
326 Definition label := nat.
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adamc@64
|
327
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adamc@64
|
328 Inductive instrs : Set :=
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adamc@64
|
329 | Const : reg -> nat -> instrs -> instrs
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|
adamc@64
|
330 | Add : reg -> reg -> reg -> instrs -> instrs
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|
adamc@64
|
331 | Halt : reg -> instrs
|
|
adamc@64
|
332 | Jeq : reg -> reg -> label -> instrs -> instrs.
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adamc@64
|
333
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adamc@67
|
334 (** [Const] stores a constant in a register; [Add] stores in the first register the sum of the values in the second two; [Halt] ends the program, returning the value of its register argument; and [Jeq] jumps to a label if the values in two registers are equal. Each instruction but [Halt] takes an [instrs], which can be read as "list of instructions," as the normal continuation of control flow.
|
|
adamc@67
|
335
|
|
adamc@67
|
336 We can define a program as a list of lists of instructions, where labels will be interpreted as indexing into such a list. *)
|
|
adamc@67
|
337
|
|
adamc@64
|
338 Definition program := list instrs.
|
|
adamc@64
|
339
|
|
adamc@67
|
340 (** We define a polymorphic map type for register keys, with its associated operations. *)
|
|
adamc@64
|
341 Section regmap.
|
|
adamc@64
|
342 Variable A : Set.
|
|
adamc@64
|
343
|
|
adamc@64
|
344 Record regmap : Set := Regmap {
|
|
adamc@64
|
345 VR1 : A;
|
|
adamc@64
|
346 VR2 : A
|
|
adamc@64
|
347 }.
|
|
adamc@64
|
348
|
|
adamc@67
|
349 Definition empty (v : A) : regmap := Regmap v v.
|
|
adamc@64
|
350 Definition get (rm : regmap) (r : reg) : A :=
|
|
adamc@64
|
351 match r with
|
|
adamc@64
|
352 | R1 => VR1 rm
|
|
adamc@64
|
353 | R2 => VR2 rm
|
|
adamc@64
|
354 end.
|
|
adamc@64
|
355 Definition set (rm : regmap) (r : reg) (v : A) : regmap :=
|
|
adamc@64
|
356 match r with
|
|
adamc@64
|
357 | R1 => Regmap v (VR2 rm)
|
|
adamc@64
|
358 | R2 => Regmap (VR1 rm) v
|
|
adamc@64
|
359 end.
|
|
adamc@64
|
360 End regmap.
|
|
adamc@64
|
361
|
|
adamc@67
|
362 (** Now comes the interesting part. We define a co-inductive semantics for programs. The definition itself is not surprising. We could change [CoInductive] to [Inductive] and arrive at a valid semantics that only covers terminating program executions. Using [CoInductive] admits infinite derivations for infinite executions. *)
|
|
adamc@67
|
363
|
|
adamc@64
|
364 Section run.
|
|
adamc@64
|
365 Variable prog : program.
|
|
adamc@64
|
366
|
|
adamc@64
|
367 CoInductive run : regmap nat -> instrs -> nat -> Prop :=
|
|
adamc@64
|
368 | RConst : forall rm r n is v,
|
|
adamc@64
|
369 run (set rm r n) is v
|
|
adamc@64
|
370 -> run rm (Const r n is) v
|
|
adamc@64
|
371 | RAdd : forall rm r r1 r2 is v,
|
|
adamc@64
|
372 run (set rm r (get rm r1 + get rm r2)) is v
|
|
adamc@64
|
373 -> run rm (Add r r1 r2 is) v
|
|
adamc@64
|
374 | RHalt : forall rm r,
|
|
adamc@64
|
375 run rm (Halt r) (get rm r)
|
|
adamc@64
|
376 | RJeq_eq : forall rm r1 r2 l is is' v,
|
|
adamc@64
|
377 get rm r1 = get rm r2
|
|
adamc@64
|
378 -> nth_error prog l = Some is'
|
|
adamc@64
|
379 -> run rm is' v
|
|
adamc@64
|
380 -> run rm (Jeq r1 r2 l is) v
|
|
adamc@64
|
381 | RJeq_neq : forall rm r1 r2 l is v,
|
|
adamc@64
|
382 get rm r1 <> get rm r2
|
|
adamc@64
|
383 -> run rm is v
|
|
adamc@64
|
384 -> run rm (Jeq r1 r2 l is) v.
|
|
adamc@64
|
385 End run.
|
|
adamc@64
|
386
|
|
adamc@67
|
387 (** We can write a function which tracks known register values to attempt to constant fold a sequence of instructions. We track register values with a [regmap (option nat)], where a mapping to [None] indicates no information, and a mapping to [Some n] indicates that the corresponding register is known to have value [n]. *)
|
|
adamc@67
|
388
|
|
adamc@64
|
389 Fixpoint constFold (rm : regmap (option nat)) (is : instrs) {struct is} : instrs :=
|
|
adamc@64
|
390 match is with
|
|
adamc@64
|
391 | Const r n is => Const r n (constFold (set rm r (Some n)) is)
|
|
adamc@64
|
392 | Add r r1 r2 is =>
|
|
adamc@64
|
393 match get rm r1, get rm r2 with
|
|
adamc@67
|
394 | Some n1, Some n2 =>
|
|
adamc@67
|
395 Const r (n1 + n2) (constFold (set rm r (Some (n1 + n2))) is)
|
|
adamc@64
|
396 | _, _ => Add r r1 r2 (constFold (set rm r None) is)
|
|
adamc@64
|
397 end
|
|
adamc@64
|
398 | Halt _ => is
|
|
adamc@64
|
399 | Jeq r1 r2 l is => Jeq r1 r2 l (constFold rm is)
|
|
adamc@64
|
400 end.
|
|
adamc@64
|
401
|
|
adamc@67
|
402 (** We characterize when the two types of register maps we are using agree with each other. *)
|
|
adamc@67
|
403
|
|
adamc@64
|
404 Definition regmapCompat (rm : regmap nat) (rm' : regmap (option nat)) :=
|
|
adamc@64
|
405 forall r, match get rm' r with
|
|
adamc@64
|
406 | None => True
|
|
adamc@64
|
407 | Some v => get rm r = v
|
|
adamc@64
|
408 end.
|
|
adamc@64
|
409
|
|
adamc@67
|
410 (** We prove two lemmas about how register map modifications affect compatibility. A tactic [compat] abstracts the common structure of the two proofs. *)
|
|
adamc@67
|
411
|
|
adamc@67
|
412 (** remove printing * *)
|
|
adamc@64
|
413 Ltac compat := unfold regmapCompat in *; crush;
|
|
adamc@64
|
414 match goal with
|
|
adamc@88
|
415 | [ H : _ |- match get _ ?R with Some _ => _ | None => _ end ] => generalize (H R); destruct R; crush
|
|
adamc@64
|
416 end.
|
|
adamc@64
|
417
|
|
adamc@64
|
418 Lemma regmapCompat_set_None : forall rm rm' r n,
|
|
adamc@64
|
419 regmapCompat rm rm'
|
|
adamc@64
|
420 -> regmapCompat (set rm r n) (set rm' r None).
|
|
adamc@64
|
421 destruct r; compat.
|
|
adamc@64
|
422 Qed.
|
|
adamc@64
|
423
|
|
adamc@64
|
424 Lemma regmapCompat_set_Some : forall rm rm' r n,
|
|
adamc@64
|
425 regmapCompat rm rm'
|
|
adamc@64
|
426 -> regmapCompat (set rm r n) (set rm' r (Some n)).
|
|
adamc@64
|
427 destruct r; compat.
|
|
adamc@64
|
428 Qed.
|
|
adamc@64
|
429
|
|
adamc@67
|
430 (** Finally, we can prove the main theorem. *)
|
|
adamc@64
|
431
|
|
adamc@64
|
432 Section constFold_ok.
|
|
adamc@64
|
433 Variable prog : program.
|
|
adamc@64
|
434
|
|
adamc@64
|
435 Theorem constFold_ok : forall rm is v,
|
|
adamc@64
|
436 run prog rm is v
|
|
adamc@64
|
437 -> forall rm', regmapCompat rm rm'
|
|
adamc@64
|
438 -> run prog rm (constFold rm' is) v.
|
|
adamc@64
|
439 Hint Resolve regmapCompat_set_None regmapCompat_set_Some.
|
|
adamc@64
|
440 Hint Constructors run.
|
|
adamc@64
|
441
|
|
adamc@65
|
442 cofix;
|
|
adamc@65
|
443 destruct 1; crush; eauto;
|
|
adamc@65
|
444 repeat match goal with
|
|
adamc@67
|
445 | [ H : regmapCompat _ _
|
|
adamc@67
|
446 |- run _ _ (match get ?RM ?R with
|
|
adamc@67
|
447 | Some _ => _
|
|
adamc@67
|
448 | None => _
|
|
adamc@67
|
449 end) _ ] =>
|
|
adamc@65
|
450 generalize (H R); destruct (get RM R); crush
|
|
adamc@65
|
451 end.
|
|
adamc@64
|
452 Qed.
|
|
adamc@64
|
453 End constFold_ok.
|
|
adamc@64
|
454
|
|
adamc@67
|
455 (** If we print the proof term that was generated, we can verify that the proof is structured as a [cofix], with each co-recursive call properly guarded. *)
|
|
adamc@67
|
456
|
|
adamc@64
|
457 Print constFold_ok.
|
|
adamc@81
|
458
|
|
adamc@81
|
459
|
|
adamc@81
|
460 (** * Exercises *)
|
|
adamc@81
|
461
|
|
adamc@81
|
462 (** %\begin{enumerate}%#<ol>#
|
|
adamc@81
|
463
|
|
adamc@81
|
464 %\item%#<li># %\begin{enumerate}%#<ol>#
|
|
adamc@81
|
465 %\item%#<li># Define a co-inductive type of infinite trees carrying data of a fixed parameter type. Each node should contain a data value and two child trees.#</li>#
|
|
adamc@81
|
466 %\item%#<li># Define a function [everywhere] for building a tree with the same data value at every node.#</li>#
|
|
adamc@81
|
467 %\item%#<li># Define a function [map] for building an output tree out of two input trees by traversing them in parallel and applying a two-argument function to their corresponding data values.#</li>#
|
|
adamc@81
|
468 %\item%#<li># Define a stream [falses] where every node has the value [false].#</li>#
|
|
adamc@81
|
469 %\item%#<li># Define a stream [true_false] where the root node has value [true], its children have value [false], all nodes at the next have the value [true], and so on, alternating boolean values from level to level.#</li>#
|
|
adamc@81
|
470 %\item%#<li># Prove that [true_falses] is equal to the result of mapping the boolean "or" function [orb] over [true_false] and [falses]. You can make [orb] available with [Require Import Bool.]. You may find the lemma [orb_false_r] from the same module helpful. Your proof here should not be about the standard equality [=], but rather about some new equality relation that you define.#</li>#
|
|
adamc@81
|
471 #</ol>#%\end{enumerate}% #</li>#
|
|
adamc@81
|
472
|
|
adamc@81
|
473 #</ol>#%\end{enumerate}% *)
|
